1. Field of the Invention
The present invention relates to a microcomputer and to a non-contact IC card using the microcomputer.
2. Description of the Related Art
FIG. 5 illustrates the construction of a conventional non-contact IC card. A ROM 2a, a RAM 2b, a transmitting circuit 3 and a receiving circuit 4 are connected to a CPU 1. A microcomputer 5 comprises these components. A data transmitting antenna 6 and a data receiving antenna 7 are respectively connected to a transmitting circuit 3 and a receiving circuit 4 of the microcomputer 5. A battery 8 and an oscillator 9 are connected to the CPU 1. Further, the entire IC card is sealed with resin or the like in order to improve environmental resistance.
The CPU 1 is supplied with a power-supply voltage from the battery 8 and with a clock signal from the oscillator 9. With these components, the CPU 1 operates based on programs previously stored in the ROM 2a. This IC card transfers data to and from the outside by using electromagnetic waves. On the one hand, when data is received, electromagnetic waves from the outside are received by the receiving antenna 7, and input into the CPU 1 after it has been demodulated into data by the receiving circuit 4. Further, data processing is performed in the CPU 1 and the data is stored as required in the RAM 2b. On the other hand, when data is transmitted, it is output from the CPU 1 to the transmitting circuit 3, and is transmitted from the transmitting antenna 6 after a carrier has been modulated by the data in the transmitting circuit 3.
FIG. 6 illustrates a circuit diagram of the receiving circuit 4. A capacitor 11 is connected to the receiving antenna 7. The receiving antenna 7 and the capacitor 11 define a resonance circuit 12 so as to selectively receive electromagnetic wave of a specific frequency. A first input of a comparator 13 for confirming whether or not data is being received is connected to the resonance circuit 12. Resistors 14, 15 for determining a reference voltage of the comparator 13 are connected to a second input of the comparator 13. The output of the comparator 13 is connected to the CPU 1 via a diode 16. Furthermore, a capacitor 17 and a discharge resistor 18 are connected to the output of the diode 16.
Further, as shown in FIGS. 7(a)-7(d), when the data of FIG. 7(a) is transmitted from an external unit to the IC card, the electromagnetic waves are received by the receiving antenna 7, and a generated voltage shown FIG. 7(c) from the receiving antenna 7 is input to the comparator 13. The data of FIG. 7(d) is demodulated in the comparator 13, and the demodulated data is input from there to the CPU 1 through the diode 16.
Since the receiving antenna 7 and the capacitor 11 comprise the resonance circuit 12, the generated voltage at the receiving antenna 7 cannot rise or fall sharply, but instead rises and falls gradually. For this reason, as shown in FIG. 7, the width of the demodulated data varies greatly owing to the magnitude of a detect level in the comparator 13 and of the generated voltage of the receiving antenna 7 and the like. This leads to a problem in that it becomes difficult to accurately receive the data. Moreover, variations in the detect level of the comparator 13 are caused by variability in the production of the IC card, and the generated voltage at the receiving antenna 7 varies greatly according to the distance between the external unit and the IC card.
Particularly, when data transmission intervals become short, despite no electromagnetic wave being received, a voltage remains in the resonance circuit 12, causing possible mis-reception.